This invention relates to a voltage-controlled oscillator.
In general, the oscillation frequency of an oscillator is determined by a resonator and also the equivalent capacitance of a bipolar-transistor circuit connected to the resonator. Thus, the oscillation frequency fluctuates with variations in the equivalent load capacitance.
Part of a Colpitts oscillator using a bipolar transistor can be realized by a semiconductor integrated circuit (IC). In such a Colpitts oscillator IC, a stray capacitance between a transistor collector and an IC substrate being a ground causes a problem. This stray capacitance includes a transistor pn junction capcitance. Thus, the stray capacitance depends on a temperature and a related voltage relative to the IC substrate. The stray capacitance composes a load capacitance connected to a resonator. Therefore, the oscillation frequency of the Colpitts oscillator fluctuates with variations in the temperature and the voltage.
Smaller-size and lower-price voltage-controlled oscillators have been demanded. A good way of satisfying this demand is to form at least part of the voltage-controlled oscillator by a semiconductor integrated circuit.